I've got an extension to a 1941 house, probably added in the 50's, that
has an attic space above it. I want to keep the rooms underneath
warmer. The attic has a wooden floor and rafters covered with
wallboard, and I've no idea what is behind any of it. Judging by the
main attic, the floor probably has some loose rock wool (?) or
fiberglass insulation without vapor retarder, well ventiliated by mice,
and I can't even guess as to the ceiling. There's no outside
ventilation to the space, it was used for storage or crude kid's
playspace. My original plan was just to lay unfaced R-30 batting down
and get on with life.

I noticed one posting here talking about the utility of moisture
retarders on attic insulation given ceiling paint (don't need facing,
paint does enough) vs. panelling (less airtight, should use facing),
and got to thinking... the extension has wood panelling on the ceilings
underneath. Should I lay down a sheet of plastic over the wood floor,
not knowing what is underneath? I know you don't normally put vapor
barriers between insulation, but could it hurt/help much here? Will it
really make much difference either way?